The preferred embodiments relate to electronic power driven systems, such as those driven with power field effect transistors (FETs).
The factors involved in certain FET power applications may conflict with one another, in terms of sufficiently sourcing power while protecting component longevity. For example, some electronically-driven power devices have high transient demands, such as at cold start-up, which tend toward requiring high current flow to meet the device (or customer) demands. However, high current flow can cause stress, damage, and fault violations to power driving circuitry, including one or more FETs. Certain prior art approaches have evolved in an effort to balance between these competing factors, but such approaches are not particularly ideal in some applications. For example, in automotive body module applications, such as energizing an incandescent bulb coil at cold temperatures, very high peak in-rush current may be required to initially drive the coil, such as current demands in the range of approximately 90 A to 100 A. Typically, a high-side power FET is used as switch to allow this much current to flow, and in order to meet the high demands, any limit on current flow must be higher than the expected demand. Hence, a level of protective circuitry may be included that disables current flow in response to instantaneous current or power exceeding a set threshold, but the application dictates a high current threshold. Such a threshold, therefore can lead to very high voltage across the FET in instances other than the in-rush event. For example, if a true short-circuit develops in the load, then large amounts of current may flow within the limit of the protective circuitry, while that current is sourced immediately to ground via the short. As another example, where the load is inductive, as can be the case for a long cable short, then there may be a sudden negative voltage spike that causes a high drain-to-source voltage across the FET that is driving the inductive load, when that FET is disabled which is a condition known as fly-back, or the FET otherwise can accumulate excessive energy that can cause stress or damage to the FET. Thus, by implementing a higher limit, then stress/damage or other violations of the safe operating area (SOA) boundary violations of the FET may occur during switch turn-on, switch turn-off, and other events.
The above-described automotive application may suffer an additional drawback if addressed with the prior art instantaneous current or power protective circuit. Specifically, the instantaneous nature of such a circuit causes a shutdown of current flow when the monitored threshold is exceeded, followed typically by a delay and retry, that is, where power is restored following the threshold-detection. However, if the current demands of the circuit rise quickly yet for a short time, the protective circuit may immediately respond by disabling current flow, then retry only to repeat the disablement, with the process causing repeated failures in sourcing current that is otherwise needed for normal operation of the application.
Given the preceding, while the prior art approaches may be acceptable in certain implementations, some applications may have requirements that are not satisfactorily met with these prior art approaches. Alternatively, such approaches may be deemed unacceptable to an electronics customer seeking to implement an application. Thus, the present inventors seek to improve upon the prior art, as further detailed below.